1. Field of the Invention
The present invention relates generally to a light diverging/converging device employable for an optical fiber communication system. More particularly, the present invention relates to a diverging ratio variable type diverging/converging device having no dependency on the wavelength of a light source wherein the end surface of an optical fiber comes in contact with the end surface(s) of an opponent optical fiber(s). Further, the present invention relates to a structural element such as a single circuit converging ferrule, a double circuit diverging ferrule or the like for a light diverging/converging device. In addition, the present invention relates to a method of producing a structural element for a light diverging/converging device such as a 1.times.2 circuit diverging unit or the like employable for a light diverging/converging device of the aforementioned type.
2. Description of the Related Art
Many kinds of light diverging/converging devices, each outputting optical power through a plurality of optical circuits branched from a single optical circuit while the initial power ratio is arbitrarily changed to 1:1 or the like, have been heretofore put in practical use for an optical fiber communication system.
With respect to each light diverging/converging device, it has been required that the diverging ratio hardly vary as a function of the wavelength of a light source, the device have few dependencies on the wavelength of the light source, the diverging ratio can selectively be determined with a high accuracy, a method of producing a structural element for the device can be practiced, and these structural elements can be produced with high efficiency on a mass production line.
Conventional light diverging devices and conventional light diverging/converging devices are typically classified by type in the following manner:
One of them is a thermally stretching type wherein two heated optical fibers are stretched to have a gradually reduced diameter while they come in close contact with each other and extend in parallel with each other in the longitudinal direction. Another one is a so-called block grinding type wherein V-shaped grooves are formed on one surface of a block, optical fibers each having a sheath removed therefrom are immovably placed in the respective V-shaped grooves with the aid of an adhesive, the optical fibers are subjected to plane grinding to such an extent that cores of the respective optical fibers are not exposed to the outside, and finally, the block thus prepared is assembled with an opponent block having the same structure as that of the first-mentioned one. It should be noted that an Evernescent effect is utilized for each of the aforementioned types.
However, the conventional diverging/converging device has a drawback in that its light diverging ratio largely varies dependent on the wavelength of a light source For this reason, the conventional diverging/converging device is not suitable for a multiple-wavelength type optical fiber communication system having a large capacity which is expected to widen its application fields in the near future.
In addition to the diverging/converging devices of the aforementioned types, a so-called optical fiber end surface connection type diverging/converging device is also known which is constructed such that end surfaces of two optical fibers longitudinally integrated with each other and each having a sheath removed therefrom on the diverging side of the device come in contact with a single optical fiber having a sheath removed therefrom on the converging side of the same.
Dependency of a diverging ratio of the optical fiber end surface connection type diverging/converging device on the wavelength of a light source is substantially equivalent to the characteristics of the optical fiber itself. For example, with respect to the wavelength of a widely used light source ranging from 800 to 1600 micronmeters, the dependency of the diverging ratio of the device on the wavelength of the light source is negligibly small. For this reason, the foregoing type light diverging/converging device is most suitably employable for the purpose of uniformly diverging and converging a plurality of multiple-wavelength signals each having a different wavelength.
Many proposals have been heretofore made with respect to a structure of a so-called optical fiber end surface connection type light diverging/converging device and a method of producing the same, as disclosed in U.S. Pat. Nos. 4,666,541 and 4,720,161.
However, the conventional light diverging/converging device as proposed in this way has the following drawbacks.
When the light diverging/converging device of the aforementioned type is produced, it is usually anticipated that there arises a diverging ratio error after completion of an assembling operation associated with a production error inherent to the optical fiber itself as well as a machining error during a machining operation for bisecting an optical fiber having a sheath removed therefrom. For this reason, it is inevitably necessary to prepare means for properly correcting the diverging ratio after completion of the assembling operation. For example, in the case of an optical fiber core having a diameter of 10 micronmeters and a diverging ratio of 5:1, the diverging ratio largely varies within the range of 14:1 to 3.5:1 when the bisectioned plane of an optical fiber is dislocated by a distance of .+-.1 micronmeter. For this reason, the diverging ratio should be corrected after completion of an assembling operation. However, each of the hitherto proposed conventional light diverging/converging devices does not take into account the aforementioned problems at all.
In the case which an optical fiber core on the output side is designated by P and optical fibers on the diverging side are designated by P1 and P2 as shown in FIGS. 9(a), (9c) and 9(d), to assure that the diverging/converging device has a diverging ratio of 50:50, the optical fibers P1 and P2 are machined in the axial direction and then longitudinally integrated with each other such that each of them is bisectioned into two halves along the cut line m which extends through a central point 0 of the optical fiber P where the ratio of the area of the machined optical fiber P1 to the area of the machined optical fiber P2 is 50:50, as shown in FIG. 9(c). In addition, when the diverging/converging device has a diverging ratio of 80:20, each of the optical fibers P1 and P2 is bisectioned into two halves along the cut line n which extends in conformity with the ratio of the area of the optical fiber P1 to the area of the optical fiber P2 corresponding to the given diverging ratio, as shown in FIG. 9(d).
A single mode optical fiber core does not exhibit a uniform light propagation mode not only at the central part but also around the outer peripheral region thereof. Especially, it has an unstable zone around the outer peripheral region. On the other hand, a multi-mode optical fiber core is constructed such that its refractive index varies from the central part to the outer peripheral region. For example, in case that the single mode optical fiber core has a diverging ratio of 80:20, it should be bisectioned into two halves along the cut plane located away from the central axis of the optical fiber by a distance of 2.7 micronmeters. For this reason, the light propagation portion of the optical fiber core becomes very unstable within the very narrow range having a width of 2.3 microns as measured from the outer diameter, resulting in a serious problem appearing from the viewpoint of the light diverging principle.
In addition, it is substantially impossible to machine an optical fiber having a sheath removed therefrom wherein the optical fiber has a different diameter corresponding to the diverging ratio to be obtained.